X-ray Variability from the Ultraluminous Black Hole Candidate X-ray Binary in the Globular Cluster RZ 2109

TL;DR

This study analyzes 16 years of X-ray observations of a globular cluster source, revealing persistent soft spectra and significant flux variability, supporting its classification as an accreting black hole.

Contribution

It provides a comprehensive long-term analysis of X-ray variability and spectral stability in a candidate black hole system within a globular cluster.

Findings

X-ray flux varies significantly over time.

Spectra are consistently dominated by a soft component.

Spectral shape remains stable despite luminosity changes.

Abstract

We present the results of long-term monitoring of the X-ray emission from the ultraluminous X-ray source XMMUJ122939.9+075333 in the extragalactic globular cluster RZ2109. The combination of the high X-ray luminosity, short term X-ray variability, X-ray spectrum, and optical emission suggest that this system is likely an accreting black hole in a globular cluster. To study the long-term behavior of the X-ray emission from this source, we analyze both new and archival Chandra and XMM-Newton observations, covering 16 years from 2000 to 2016. For all of these observations, we fit extracted spectra of RZ2109 with xspec models. The spectra are all dominated by a soft component, which is very soft with typical fit temperatures of T $\simeq$ 0.15 keV. The resulting X-ray fluxes show strong variability on short and long timescales. We also find that the X-ray spectrum often shows no significant change even with luminosity changes as large as a factor of five.